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Current time:0:00Total duration:10:40

Video transcript

so we've talked about angiotensin 2 and we know that angiotensin 2 is a pretty small hormone it's only about eight amino acids and so I'm going to draw it that way of eight little balls representing one amino acid per tang a little ball almost like pearls on a necklace and they're floating through this blood vessel and they're headed to many different targets so these little molecules are headed to various organs and so let's talk about what those organs might be so one target for sure is the blood vessel so we have the blood vessel here and in the blood vessel wall we have smooth muscle and the angiotensin hormone actually gets that smooth muscle to constrict and so that's called vaso constriction and it's actually easy to remember this because if you think about the word angiotensin it's literally angio meaning blood vessel and tension you can think of making tense so it's making the blood vessel tense and constrict down and we know that if you cause vasoconstriction you're going to actually increase resistance because that's how resistance works in tubes and so if you're increasing resistance try to keep in mind that formula that we talked about way back when for blood pressure Delta P equals Q times R and now we're finally kind of seeing how this formula is useful and so if we talk about p on the arterial side minus p on the venous side that would be the change in pressure that would be Delta P and that equals Q and this Q is actually going to be a couple things it's going to be stroke volume times heart rate and so that's the flow and all that times resistance and so and I should should make this very clear so you're not confused by what I'm writing here sometimes my penmanship gets a little bit wacky but you've got this is your flow so you have these this increase in resistance and you can see that if I tell you that your venous pressure over here is really not going to change a whole heck of a lot then if you can increase your resistance then you can definitely see how you would increase your arterial pressure so it makes perfect sense using the formula and you can see now how angiotensin 2 accomplishes that so that's not oh and actually the last thing I should mention before I move on is that this is actually a pretty rapid response so very quickly the blood vessels will start constricting if angiotensin 2 is around so now another target organ would be the kidneys and so here's a little kidney here and this kidney is going to be affected by angiotensin 2 very slowly by comparison so it's actually more of a slow response and what actually happens is you get sodium reabsorption so sodium reabsorption and as the kidneys are reabsorbing the the sodium they actually also pick up water and so as the blood starts filling up with more sodium and more water that you're not peeing out because you're of course reabsorbing it from what would otherwise have been urine you end up having very concentrated urine and your blood ends up getting all the salt and water and your stroke volume goes up so your stroke volume increases and you can see from that equation that we just drew that if your stroke volume goes up then again your arterial blood pressure would go up as well so here's a double check for that so now the stroke volume goes up annual resistance goes up your part your arterial pressure is definitely going to start going up so angiotensin affects two different target organs and actually it's not even done there it continues to affect other things so it even has an effect on the pituitary gland so this is your pituitary gland and the pituitary gland is actually in charge of releasing hormones of its own and so when it gets a signal from angiotensin - it'll start sending off its own hormone called ADH and ADH is antidiuretic hormone and actually instead of so it'll definitely cause vasoconstriction of the blood vessels just like angiotensin 2 did but instead of that sodium reabsorption this ADH actually causes water reabsorption water reabsorption now the effect for blood pressure in many ways is going to be similar because if you're reabsorbing water again your stroke volume will go up and if your stroke volume goes up your arterial pressure goes up so at the end of the day your pressure will still go up but it's slightly different because its water reabsorption vs. salt reabsorption and we'll talk about the difference momentarily but before I get to that the last target organ I want to mention is another gland called the adrenal gland and the adrenal gland is literally sitting on top of the kidneys and that's why it's called adrenal and the adrenal gland is going to send off its own hormone called aldosterone aldosterone and aldosterone is going to affect the kidneys and just like the angiotensin ii aldosterone is going to cause salt reabsorption and that's the main kind of thing that it does and this salt reabsorption is going to lead to more water reabsorption and increase in stroke volume so you can see how increase in resistance and increase in stroke volume is how our body is going to get our blood pressure back in control now I want to talk about one thing which is in a little bit more detail which is this whole salt versus water reabsorption issue so both of them increase stroke volume so you might be wondering you know what is the difference in and why did I talk about the two separately so let me let me get to that now let's do sodium or salt on this side I'll write sodium and on this side I'll write water we'll talk about sodium first so if you have your nephron here this is what's going to eventually lead to urine you have little cells here lining it and you have them on both sides I'm just going to focus on one side for simplicity and you have a blood vessel let's say right here and so these cells are going to help to reabsorb stuff that's otherwise going to go into the into the urine and so one strategy for getting water back let's say you want to reabsorb water which is what you want to do if you want to increase your blood pressure one strategy for getting water back would be to pull out salt because you know that if you pull out salt through osmosis water is going to follow right that's a pretty good strategy getting water back that would work but the assumption and this is very very important the assumption is that this barrier right here is permeable to water permeable to water and so if it is permeable to water then this sodium reabsorption strategy works now let's imagine for a second that you try this and it's actually not permeable to water what would happen well if you didn't have that permeability I'm going to redraw it over here then when you try to bring the salt over and let's say you have your blood vessel again over here you try to bring your salt over the moment that the water tries to follow it's going to bounce off right it's going to do this and bounce right off and so it's not going to work so you got to try something different and so that's exactly what happens is that in areas where you don't have permeability to water so let's say this is not permeable not permeable to water you need a new strategy and the strategy in a ways is very very simple it's well if it's not permeable to water why not forget about reabsorbing salt for the moment why not just do something like and create little channels and so that's exactly what what happens is you create these little channels and water I can just go through it so basically you make it permeable by creating channels and say okay well that's the better strategy for getting water in this case so if it's initially not permeable to water put on a bunch of water channels and force that water or allow that water maybe not force is not the right word allow that water to get through using your own channels and so that's basically what the difference is and so if you look at ADH versus the other two hormones aldosterone and angiotensin 2 ADH is using the water channel approach because here all right in red here where this works the water is usually not permeable so I mean the the nephron is not permeable normally to water and so that's why ADH throws in a bunch of water channels and aldosterone an angiotensin work in areas of the nephron that are permeable that are permeable to water and so that's why their salt strategy works pretty well but you can see now that in both situations the key is getting water back either doing it through a salt gradient or doing it through getting a bunch of water channels in there in both situations you would increase your stroke volume